Electrophotographic photoreceptors are required to have, for example, the following basic properties: (1) they should be able to be charged in the dark to a proper potential; (2) dissipation of the charge in the dark should be little; and (3) they should be able to quickly dissipate the charge upon exposure to light.
Conventionally used electrophotographic photoreceptors employing inorganic substances such as selenium, cadmium sulfide and zinc oxide have many advantages. At the same time, however, they also have various disadvantages. For example, selenium photoreceptors, which are being widely used and fully satisfy the above requirements (1) to (3), are disadvantageous in that the conditions for their preparation are complicated, their production costs are high, they lack flexibility so that they are difficult to prepare into a belt configuration, and they should be carefully handled because of their sensitiveness to heat and mechanical shock. Cadmium sulfide photoreceptors and zinc oxide photoreceptors, which employ cadmium sulfide or zinc oxide dispersed in resin binders, cannot be readily used in a repetitive cyclic operation since they lack smoothness and are poor in mechanical properties such as hardness, tensile strength and abrasion resistance.
In recent years, electrophotographic photoreceptors employing various organic substances have been suggested in order to eliminate the disadvantages of the conventional photoreceptors employing the above mentioned inorganic substances, and some of these have been put to practical use. For example, there may be mentioned an electrophotographic photoreceptor comprising poly-N-vinylcarbazole and 2,4,7-trinitrofluoren-9 one (U.S. Pat. No. 3,484,237), an electrophotographic photoreceptor comprising poly-N-vinylcarbazole sensitized with a pyrylium salt-based dye (JP B-48-25658), an electrophotographic photoreceptor comprising an organic pigment as the principle component (JP-A-47-37543), and an electrophotographic photoreceptor in which the principle component is a co-crystalline complex consisting of a dye and a resin (JP-A-47-10785). (The terms "JP-A" and "JP-B" as used herein mean an "unexamined published Japanese patent application" and an "examined Japanese patent publication" respectively.)
Furthermore, there has also been proposed an electrophotographic photoreceptor comprising copper phthalocyanine dispersed in a resin (JP-B-52-1667).
Although these organic electrophotographic photoreceptors possess somewhat improved mechanical properties and flexibility as compared with the aforementioned inorganic electrophotographic photoreceptors, they generally have poor photosensitivities and are unfit for use in repetitive operations. Hence, these organic photoreceptors cannot fully meet the requirements for electrophotographic photoreceptors.
Meanwhile, the process of photoconduction in electrophotographic photoreceptors consists of
(1) a step in which the photoreceptors generate an electrical charge upon exposure to light, and
(2) a step in which the photoreceptors transport the charge.
As an example in which the steps (1) and (2) proceed in a single material, there may be mentioned the selenium photoconducting plate. On the other hand, a well known example in which the steps (1) and (2) proceed in separate materials is a photoconductor employing the combination of amorphous selenium and poly-N-vinylcarbazole. The function-divided electrophotographic photoreceptor in which the steps (1) and (2) are allowed to proceed in separate materials is advantageous in that materials therefor can be selected from a wide range and, hence, the electrophotographic properties such as sensitivity and potential acceptability of the resulting electrophotographic photoreceptors can be improved, and further that materials advantageous for preparing coatings of electrophotographic photoreceptors can be selected from wide ranges.
Many types of such function-divided electrophotographic photoreceptors have so far been proposed. Recently, however, extensive studies are being conducted in order to develop a layered photoreceptor comprising two layers consisting of a charge generating layer which serves to absorb light and generate a charge and a charge transporting layer which serves to transport the charge generated.
The charge generating layer is basically composed of a charge generating material which absorbs light and generates a charge carrier, and a resin binder. In electrophotographic processes, the charge generating layer is required to have high photosensitivity, the fluctuation in potential is required to be small throughout repetitive use, and further the charge carrier generated in the charge generating layer should be efficiently injected into the charge transporting layer. The resin binder containing the charge generating material dispersed therein greatly affects the transport of generated charge carriers depending upon the chemical structure, molecular weight, purity, etc. of the resin binder.
As resin binders for charge generating layers, a polyester resin is being used which is combined with an azo pigment (JP-A-54-22834). Further, there have also been proposed, for example, a hydroxypropyl cellulose resin (JP-A-57-169754), a resin of a fatty acid ester with cellulose (JP-A-58-166353), an acrylic resin (JP-A-58-192040), a polyvinyl butyral resin used in combination with a polyamide undercoat (JP-A-58-30757), a linear polyester resin combined with an undercoat of a polyamide copolymer (JP-A-58-93739), and a phenoxy, polyvinyl formal or ethyl cellulose resin combined with an alcohol-soluble nylon undercoat (JP-A-60-196766, JP-A-60-202448 and JP-A60-202449).
However, electrophotographic photoreceptors employing these resin binders for their charge generating layers are still insufficient in sensitivity and durability and, hence, a further improvement has been demanded.